B-1 Visa

This is discouraged as it permanently terminates STEM OPT status. The government may question your intent. Consider alternative compensation or cap-exempt H-1B roles instead.

 Once an I-140 is approved and held for 180 days, the priority date is locked, and the applicant can extend an H-1B infinitely with any employer. To bridge a gap between roles, filing for a B-1 visa with an explicit intent of job hunting is a suggested strategy, rather than switching to an F-1 unless it represents a logical career progression.

A standard practice is to include a notarized letter inside the new H-1B petition package. The letter should explicitly state that if the H-1B is approved with status, the pending B-2 should be withdrawn; otherwise, B-2 processing should continue. 

This is strongly discouraged. Entering the US on a visitor visa with the preconceived intent to convert to student status is considered misrepresentation and may result in a permanent bar from the United States.

No. The "90-day rule" is a misconception; USCIS always evaluates the intent behind your entry. Entering with the intention to change status can be viewed as misrepresentation. It is generally safer to process your visa stamp outside the US.

This is "up in the air" and varies by case. Filing for B-1 might "dilute" the F-1 filing, and the regulations could potentially hurt the applicant later.

Getting an H-4 visa stamping abroad is much cleaner. Applying for a change of status carries the risk that the government might claim she had "preconceived intent" to stay long-term while entering on a tourist visa.